Multi-functional electronic components such as SOP's (small outline packages) and QFP's (quad flat packages) have leads on opposite long sides or surrounding four sides of their relatively flat bodies. However, with an increase in integration in a single semiconductor chip, the number of leads capable of being arranged in such a component was sometimes insufficient to allow the chip to exert all of its possible functions. Thus, electronic components having a much larger number of leads than attainable with SOP's and QFP's were desired. An increase in the number of leads was realized by a technique in which leads are arranged in a grid array on the back surface of a substrate on which a semiconductor chip is mounted to form an electronic component, i.e., on the surface of the substrate facing away the semiconductor chip. Examples of such electronic components having a large number of leads on the back surface of their substrates are BGA's (ball grid array packages) and CSP's (chip size packages).
Normally a BGA or CSP (hereinafter referred to merely as a BGA for simplicity) is mounted on a printed (wiring) board by use of solder balls, which are fine balls of a solder alloy. In practice, it is impossible to dispense and position solder balls at the time of mounting a BGA on a printed board. Therefore, solder balls are normally secured on the back surface of the substrate of a BGA by making them adhere to the electrode pads or lands formed on that surface of the BGA substrate to form solder bumps in a grid array. A BGA having the thus formed solder bumps is soldered to a printed board typically by applying a solder paste to the printed board, bringing the solder bumps of the BGA into contact with the surface of the printed board on which the solder paste is applied, and heating the resulting assembly in a reflow furnace so as to melt the solder paste and the solder bumps for soldering.
Solder balls which have conventionally been used to form bumps on BGA's are made of an Sn—Pb alloy which has an approximately eutectic composition of about 63% by weight of Sn and a balance of Pb and which has a melting point of 183° C. Sn—Pb alloys are excellent as a solder since they have low melting temperatures and good wetting power in a molten state, so they minimize the occurrence of soldering defects.
Recently, however, the use of Sn—Pb solders has been disfavored due to the toxic nature of Pb. When waste electronic devices are disposed of, they are often disassembled to remove plastic and metallic parts for recycling. Printed boards on which electronic components are mounted are not adapted to recycling since plastic and metallic portions are combined complicatedly therein, so they are removed from waste electronic devices, shredded, and buried in the ground. When rain which has been acidified (i.e., acid rain) contacts the shredded printed boards buried in the ground, the lead (Pb) in the Sn—Pb solder may be dissolved out and contaminate underground water. If a human or animal continues to drink lead-containing water for many years, there is a concern of lead accumulating in its body to cause lead poisoning.
Accordingly, it has been recommended in the art to use a “lead-free” solder, which is completely free from lead, in soldering of electronic components.
Most lead-free solders are made of Sn-based alloys such as Sn—Ag, Sn—Cu, Sn—Bi, and Sn—Zn alloys which may optionally contain one or more additional elements such as those selected from Ag, Cu, Zn, In, Ni, Cr, Fe, Ge, and Ga. Thus, there are a wide variety of lead-free solders which have their respective advantages and disadvantages, and an appropriate lead-free solder is selected depending on the application.
For lead-free solder balls which are used to form solder bumps on BGA's, an Sn—Ag—Cu alloy is suitable in terms of properties such as solderability, bonding strength, and thermal fatigue resistance. In particular, an Sn-3Ag-0.5Cu alloy (containing 3% Ag, 0.5% Cu, and a balance of Sn on a weight basis) having a liquidus temperature of about 220° C. is mostly used.
However, solder balls made of an Sn-3Ag-0.5Cu alloy have a problem that they often have surface defects such as shrinkage cavities and seams, which are formed while the solder balls are solidified during their production. Shrinkage cavities are pores opening onto the surface of a solder ball and extending deep inside the balls, while seams are streaky surface irregularities like wrinkles (protrusions and indentations) found numerously on the surface of a solder ball.
With solder balls having shrinkage cavities, the pore-like shrinkage cavities are closed by molten solder when the solder balls are heated to melt for the first time, i.e., so as to form solder bumps on the back surface of a BGA substrate, thereby leaving air confined within the closed shrinkage cavities. Thus, the resulting solder bumps have closed air cells therein, and they may form soldered joints having voids during soldering of the BGA to a printed board, thereby causing the joints to have a decreased bonding strength.
Solder balls having seams cause a problem when they are delivered to the back surface of a BGA substrate and located on each land formed on the back surface. In the production of BGA's, the delivery of solder balls is normally performed using a ball feeder which includes a suction plate to grasp solder balls, but seams on the surface of solder balls may prevent the balls from being grasped by the suction plate as described more fully below.
The suction plate of a ball feeder has through holes with a diameter slightly smaller than that of solder balls to be grasped thereby. These holes are situated in exactly the same grid array as that of the lands formed on the back surface of a BGA substrate on which solder bumps are to be formed. The interior of the suction plate is evacuated to generate a suction force sufficient to grasp solder balls in all the holes. The suction plate which grasps solder balls in all the holes is then moved over a BGA substrate placed upside down (with its back surface facing upward). The BGA substrate has been treated by applying an adhesive soldering flux to the grid array spots (electrodes or lands) of its back surface on which solder bumps are to be formed. After the suction plate is positioned so that its holes coincide with the grid array spots of the BGA substrate, the suction plate is moved toward the BGA substrate until the solder balls grasped by the suction plate come into contact with the flux on the BGA substrate. The solder balls are then released by an appropriate technique such as injection of air through the holes of the suction plate or application of an impact to the suction plate, thereby causing the solder balls to adhere to the flux. Thereafter, the BGA substrate having solder balls placed thereon is heated in a reflow furnace or similar heating device to melt the soldering flux and the solder balls and form solder bumps in a grid array on the substrate.
If solder balls having seams are used in the above-described process, it cannot be guaranteed that they are grasped by all the holes of the suction plate, since air can pass through the gaps formed between the balls and holes due to the irregular surfaces of the balls having seams, and thus the suction force generated by the suction plate is attenuated. As a result, some holes of the suction plate may be vacant by a failure to grasp or maintain a solder ball in the holes, and thus the ball feeder may fail to deliver solder balls to the spots of the BGA substrate corresponding to the vacant holes, resulting in the production of a BGA having no solder bumps in some spots after heating is performed to melt the solder balls. If a BGA is missing a soldering bump even in only one spot in its grid array, it will not be able to perform its function successfully. For this reason, it is critical that solder balls be free from seams.
Solder balls having seams cause another problem during the inspection of solder balls performed in the above-described process to confirm that a suction plate of a ball feeder has a solder ball in each hole of the plate before solder balls are delivered on a BGA substrate, or subsequently to confirm that the BGA substrate has a solder ball on each spot of its grid array before the substrate is heated for soldering. Such inspection is normally carried out by image processing using a photo detector which detects the light reflected by a solder ball as an indication of the presence of a ball. Seams on a solder ball may diffuse the reflected light to such an extent that the reflected light reaching the photo detector is insufficient for detection of the ball. As a result, although a solder ball is actually present in a hole of the suction plate or on a spot of the BGA substrate, the photo detector may be unable to detect the presence of the ball and mistakenly determine the hole or spot to be vacant.